Improve error handling

After a few years, the Uiml.net codebase has grown and been patched upon by different people. This had as a result that the code is not always consistent, clear or efficient. An important problem however is that it is difficult to debug a renderer, knowing what exactly went wrong. From the beginning, we had the convention to return null values, instead of throwing exceptions (in order to keep the renderer going after some mistakes). We should add a number exceptions (e.g. ParseException, RenderException, etc.) and throw them whenever something unexpected occurs, instead of always trying to continue.